It responds to me moving the mouse around, but it won't let me look more than a few pixels before snapping back to the center. If I had to hazard a guess, there's something wrong with how your mouse look is calculated in relation to how the mouse is locked in relation to locking the cursor inside the display, but without the source code I don't know for sure.

I've got three different controllers plugged in, so that might have something to do with it even though I can't use them to play the game. Either way, I'd urge you to fix this immediately.